The Python Debugger Pdb
=======================
To use the debugger in its simplest form:
>>> import pdb
>>> pdb.run('<a statement>')
The debugger's prompt is '(Pdb) '. This will stop in the first
function call in <a statement>.
Alternatively, if a statement terminated with an unhandled exception,
you can use pdb's post-mortem facility to inspect the contents of the
traceback:
>>> <a statement>
<exception traceback>
>>> import pdb
>>> pdb.pm()
The commands recognized by the debugger are listed in the next
section. Most can be abbreviated as indicated; e.g., h(elp) means
that 'help' can be typed as 'h' or 'help' (but not as 'he' or 'hel',
nor as 'H' or 'Help' or 'HELP'). Optional arguments are enclosed in
square brackets. Alternatives in the command syntax are separated
by a vertical bar (|).
A blank line repeats the previous command literally, except for
'list', where it lists the next 11 lines.
Commands that the debugger doesn't recognize are assumed to be Python
statements and are executed in the context of the program being
debugged. Python statements can also be prefixed with an exclamation
point ('!'). This is a powerful way to inspect the program being
debugged; it is even possible to change variables or call functions.
When an exception occurs in such a statement, the exception name is
printed but the debugger's state is not changed.
The debugger supports aliases, which can save typing. And aliases can
have parameters (see the alias help entry) which allows one a certain
level of adaptability to the context under examination.
Multiple commands may be entered on a single line, separated by the
pair ';;'. No intelligence is applied to separating the commands; the
input is split at the first ';;', even if it is in the middle of a
quoted string.
If a file ".pdbrc" exists in your home directory or in the current
directory, it is read in and executed as if it had been typed at the
debugger prompt. This is particularly useful for aliases. If both
files exist, the one in the home directory is read first and aliases
defined there can be overridden by the local file. This behavior can be
disabled by passing the "readrc=False" argument to the Pdb constructor.
Aside from aliases, the debugger is not directly programmable; but it
is implemented as a class from which you can derive your own debugger
class, which you can make as fancy as you like.
Debugger commands
=================
h(elp)
Without argument, print the list of available commands.
With a command name as argument, print help about that command.
"help pdb" shows the full pdb documentation.
"help exec" gives help on the ! command.
w(here) [count]
Print a stack trace. If count is not specified, print the full stack.
If count is 0, print the current frame entry. If count is positive,
print count entries from the most recent frame. If count is negative,
print -count entries from the least recent frame.
An arrow indicates the "current frame", which determines the
context of most commands. 'bt' is an alias for this command.
d(own) [count]
Move the current frame count (default one) levels down in the
stack trace (to a newer frame).
u(p) [count]
Move the current frame count (default one) levels up in the
stack trace (to an older frame).
b(reak) [ ([filename:]lineno | function) [, condition] ]
Without argument, list all breaks.
With a line number argument, set a break at this line in the
current file. With a function name, set a break at the first
executable line of that function. If a second argument is
present, it is a string specifying an expression which must
evaluate to true before the breakpoint is honored.
The line number may be prefixed with a filename and a colon,
to specify a breakpoint in another file (probably one that
hasn't been loaded yet). The file is searched for on
sys.path; the .py suffix may be omitted.
tbreak [ ([filename:]lineno | function) [, condition] ]
Same arguments as break, but sets a temporary breakpoint: it
is automatically deleted when first hit.
cl(ear) [filename:lineno | bpnumber ...]
With a space separated list of breakpoint numbers, clear
those breakpoints. Without argument, clear all breaks (but
first ask confirmation). With a filename:lineno argument,
clear all breaks at that line in that file.
disable bpnumber [bpnumber ...]
Disables the breakpoints given as a space separated list of
breakpoint numbers. Disabling a breakpoint means it cannot
cause the program to stop execution, but unlike clearing a
breakpoint, it remains in the list of breakpoints and can be
(re-)enabled.
enable bpnumber [bpnumber ...]
Enables the breakpoints given as a space separated list of
breakpoint numbers.
ignore bpnumber [count]
Set the ignore count for the given breakpoint number. If
count is omitted, the ignore count is set to 0. A breakpoint
becomes active when the ignore count is zero. When non-zero,
the count is decremented each time the breakpoint is reached
and the breakpoint is not disabled and any associated
condition evaluates to true.
condition bpnumber [condition]
Set a new condition for the breakpoint, an expression which
must evaluate to true before the breakpoint is honored. If
condition is absent, any existing condition is removed; i.e.,
the breakpoint is made unconditional.
(Pdb) commands [bpnumber]
(com) ...
(com) end
(Pdb)
Specify a list of commands for breakpoint number bpnumber.
The commands themselves are entered on the following lines.
Type a line containing just 'end' to terminate the commands.
The commands are executed when the breakpoint is hit.
To remove all commands from a breakpoint, type commands and
follow it immediately with end; that is, give no commands.
With no bpnumber argument, commands refers to the last
breakpoint set.
You can use breakpoint commands to start your program up
again. Simply use the continue command, or step, or any other
command that resumes execution.
Specifying any command resuming execution (currently continue,
step, next, return, jump, quit and their abbreviations)
terminates the command list (as if that command was
immediately followed by end). This is because any time you
resume execution (even with a simple next or step), you may
encounter another breakpoint -- which could have its own
command list, leading to ambiguities about which list to
execute.
If you use the 'silent' command in the command list, the usual
message about stopping at a breakpoint is not printed. This
may be desirable for breakpoints that are to print a specific
message and then continue. If none of the other commands
print anything, you will see no sign that the breakpoint was
reached.
s(tep)
Execute the current line, stop at the first possible occasion
(either in a function that is called or in the current
function).
n(ext)
Continue execution until the next line in the current function
is reached or it returns.
unt(il) [lineno]
Without argument, continue execution until the line with a
number greater than the current one is reached. With a line
number, continue execution until a line with a number greater
or equal to that is reached. In both cases, also stop when
the current frame returns.
j(ump) lineno
Set the next line that will be executed. Only available in
the bottom-most frame. This lets you jump back and execute
code again, or jump forward to skip code that you don't want
to run.
It should be noted that not all jumps are allowed -- for
instance it is not possible to jump into the middle of a
for loop or out of a finally clause.
r(eturn)
Continue execution until the current function returns.
retval
Print the return value for the last return of a function.
run [args...]
Restart the debugged python program. If a string is supplied
it is split with "shlex", and the result is used as the new
sys.argv. History, breakpoints, actions and debugger options
are preserved. "restart" is an alias for "run".
c(ont(inue))
Continue execution, only stop when a breakpoint is encountered.
l(ist) [first[, last] | .]
List source code for the current file. Without arguments,
list 11 lines around the current line or continue the previous
listing. With . as argument, list 11 lines around the current
line. With one argument, list 11 lines starting at that line.
With two arguments, list the given range; if the second
argument is less than the first, it is a count.
The current line in the current frame is indicated by "->".
If an exception is being debugged, the line where the
exception was originally raised or propagated is indicated by
">>", if it differs from the current line.
ll | longlist
List the whole source code for the current function or frame.
a(rgs)
Print the argument list of the current function.
p expression
Print the value of the expression.
pp expression
Pretty-print the value of the expression.
whatis expression
Print the type of the argument.
source expression
Try to get source code for the given object and display it.
display [expression]
Display the value of the expression if it changed, each time execution
stops in the current frame.
Without expression, list all display expressions for the current frame.
undisplay [expression]
Do not display the expression any more in the current frame.
Without expression, clear all display expressions for the current frame.
interact
Start an interactive interpreter whose global namespace
contains all the (global and local) names found in the current scope.
alias [name [command]]
Create an alias called 'name' that executes 'command'. The
command must *not* be enclosed in quotes. Replaceable
parameters can be indicated by %1, %2, and so on, while %* is
replaced by all the parameters. If no command is given, the
current alias for name is shown. If no name is given, all
aliases are listed.
Aliases may be nested and can contain anything that can be
legally typed at the pdb prompt. Note! You *can* override
internal pdb commands with aliases! Those internal commands
are then hidden until the alias is removed. Aliasing is
recursively applied to the first word of the command line; all
other words in the line are left alone.
As an example, here are two useful aliases (especially when
placed in the .pdbrc file):
# Print instance variables (usage "pi classInst")
alias pi for k in %1.__dict__.keys(): print("%1.",k,"=",%1.__dict__[k])
# Print instance variables in self
alias ps pi self
unalias name
Delete the specified alias.
debug code
Enter a recursive debugger that steps through the code
argument (which is an arbitrary expression or statement to be
executed in the current environment).
q(uit) | exit
Quit from the debugger. The program being executed is aborted.
(!) statement
Execute the (one-line) statement in the context of the current
stack frame. The exclamation point can be omitted unless the
first word of the statement resembles a debugger command, e.g.:
(Pdb) ! n=42
(Pdb)
To assign to a global variable you must always prefix the command with
a 'global' command, e.g.:
(Pdb) global list_options; list_options = ['-l']
(Pdb)© Python Software Foundation. PSF License Version 2 · Official docs ↗